Single phase motor blowing run capacitor

Full explanation follows...with the very last line being the actual questions .


I have an old SIP Belt drive air compressor, early 1990's.


A few months back the bottom end of the pump seized. it locked up and fuse tripped.
Stripped the pump last month, sorted out the bottom end and pump is now good as new.


The motor is a 1.5 -2.0 HP Single phase 240 volt domestic electric motor . In the UK, where standard single phase is 240 v
Motor is a twin capacitor type, 200MFD start cap, 30 MFD run capacitor.
Motor is wired as per the LHS diagram in the attached photo.

When putting it back on the compressor, I noticed that the run capacitor had blown.
Before reconnecting, the drive belt I disconnected the run capacitor, and briefly spun it up.
Motor ran just fine.

I went to a local motor rewind company this morning and bought a new 30 MFD run capacitor.
At least that is what I asked for..


I have read this thread http://www.practicalmachinist.com/v...ifference-between-run-start-capacitor-102509/
and also posted there in the hope someone who know may come over to my thread here and give some advice


So I fitted the new capacitor, and motor ran up.

Connect the drive belt back up ...motor ran.

For about 30 seconds..still no real load on the pump...tank pressure was still sub 10 psi. I have over 400 litre capacity and it had barely begun to fill. Pump still spins free..it has not seized again or anything like that!

At that point I just walked away, will go back later today.



So why would the capacitor blow again?...have the motor rewind people simply given me the wrong capacitor type? or could there be something wrong with the motor windings?

What should I test for? shorts ? but I guess if there was anything like that, the motor would not have run to start with and I'd have blown the fuse instantly.

Does the motor come up to speed? If not, a check of the centrifugal switch may be in order.

Hopefully someone who really knows will help the OP out, but here are some observations and speculations that may help.

I am familiar with capacitor start-and-run motors mostly as cheap, amazingly small-for-the-rating units from the other side of the world. I assume that the run cap allows the start winding to continue to assist the main,after the motor is up to speed. Motor in service keeps running after run cap dies unnoticed, but overloaded, then very soon burns up.
So, despite the reputable British make of the motor in question, I would be tempted to look for a cap-start, induction-run motor as a replacement, even if the original motor is for ther moment running.

Early failure of your new cap may be because it has the right mfd size,but not voltage rating. Caps used on 220VAC here commonly carry a 330V rating.

Also possible that one winding failed open when the pump locked, and now motor fails to get up to speed therefore start switch does not drop out.....no wait, that would cook the "start" cap, not the "run".

I do not know what sort of problem would kill a suitably-rated "run" cap. Back to hoping someone who knows will step up.

Hi Guys, thanks for the quick replies.

Well the motor 'seems' to come up to speed, but without a laser tacho I can not be sure...but it sure seems to be up to the correct speed, just by ear, it sound sleek it used to.

But, as far as I am aware from reading today, the start cap is switched totally out of circuit by the centrifugal switch once the motor is up to speed.

I found the cause of the cap blowing..it was the wrong voltage. they gave me a 220VAC cap.. I did ask for a Run cap, I spook to the boss earlier in the day and asked for a run cap, and asked for one when I went to collect. Trouble was the store man gave me a 220 start cap instead.

I went back and said to the boss..."
Look... I asked for a Run Cap...and from what I have been reading I'd need a voltage higher than 240 volt..1.41 - 1.56 times that voltage. "

"Yes he said that is correct.....".....

then I showed him the cap the storeman gave me..and he replaced it ..no question.



I fitted the new cap ..and motor now runs fine, left it running for 10 minutes...but ...and there is always an issue....
I now have probably found out why the compressor seized in the first place.

The pistons and bore of the pump...looked perfectly serviceable, no scoring , no visible 'ring ridge'..so I rebuilt it last week, with repaired crank and big end bearing. But when running it took an hell of an age to get to 75 psi.

I let out air till the pump kicked in again, and walked away. The came back about 30 minutes later....ummm pump still running . I could here it...went to see what pressure was up to and check for leaks. I could smell the pump from walking in the compressor room door. Spit on the pump cylinder head and it sizzled...not good.

So at a guess, that is why the big end failed last time..low oil in conjunction with massive overheating, heat travelled down the con rod..hence the melted white metal and the cod rod 'welded ' to the crank.


Big question now...Do I just buy a new compressor completely...or just buy a new belt driven pump, to fit to the old unit ? the motor seems OK.


Edit...problem solved...ordered a new pump...

http://www.sipuk.co.uk/sip-02168-pump-unit-with-fly-wheel-b2800b-f400b.html